The disclosure of Japanese Patent Application No. 2001-110580 filed on Apr. 9, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a control apparatus for an automatic transmission, and more particularly to an art for smoothly performing a speed-change operation requiring phasic changeover of the engagement and release of engagement elements between speed-change stages.
2. Description of Related Art
As is well known, an automatic transmission is designed to change over power-transmission paths extending through speed-change elements composed of planetary gears by engaging or releasing frictional engagement elements and to establish a plurality of speed-change stages by changing a gear ratio. For the purpose of performing the engagement and release of the engagement elements during a speed-change operation through the simplest hydraulic-pressure control while inhibiting generation of a speed-change shock, operations of the engagement elements for upshift or downshift are generally performed as follows. That is, basically, for a plurality of engagement elements or a single engagement element which are or is engaged to establish a certain speed-change stage, another one of the engagement elements is engaged additionally, or one engagement element that is engaged is released. A so-called changeover operation of the engagement elements is performed if it is inevitable for reasons of the structure of a gear train. That is, while the engagement elements that are engaged are released, the other engagement elements are engaged.
In recent years, automatic transmissions have developed a tendency to adopt a multiple-stage structure in response to a demand for an improvement in driveability and energy conservation based on a reduction in the amount of fuel consumed. The tendency to adopt a multiple-stage structure in automatic transmissions is realized, in general, by adding a speed-increasing or speed-reducing stage based on an overdrive or underdrive gear to a speed-change mechanism composed of a multiple-stage planetary gear set. As an alternative, however, there is also, as disclosed in Japanese Patent Application No. 4-219553, a multiple-stage structure achieved by two systems, namely, high and low systems for inputting power to a planetary gear set of Ravigneaux type.
In the aforementioned gear train having a multiple-stage structure, the possibilities of selecting speed-change stages suited for a running condition of the vehicle are widened. Therefore, the changeover operation of engagement elements is not limited to a simple changeover of two elements but may be a complicated changeover of four elements if necessary. A so-called skip speed-change operation for shifting to a specific one of a multitude of speed-change stages at a stretch is an example requiring such a changeover of four elements. Especially in performing multiple changeover, as represented by such changeover of four elements, if the engagement elements are engaged or released all at once, it is difficult to grasp the behavior of the elements in the speed-change mechanism individually. As a result, it becomes substantially impossible to perform control. Hence, a multiple changeover speed-change operation requires control for engaging or releasing the engagement elements in a certain order, whereby it becomes a requirement to perform speed-change operations in a phasic manner.
Because the engagement elements are engaged or released in a certain order, the following problems result. That is, phasic occurrence of a shock is likely during a multiple changeover speed-change operation, and the interval between speed-change operations tends to increase. The phasic occurrence of a shock during a speed-change operation or an increase in the interval between speed-change operations causes an undesirable feeling of incongruity to a driver. Timings for engaging or releasing the engagement elements are crucial in solving such problems. However, for reasons of manufacturing variations among individual automatic transmissions or operational states of the vehicle, it is difficult to perfectly match the timings for engaging or releasing the engagement elements.
It is thus an objective of the invention to provide a speed-change control apparatus for an automatic transmission capable of preventing an increase in the interval between speed-change operations while eliminating phasic occurrence of a shock during a multiple changeover speed-change operation.